In a typical cellular radio system, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a radio access network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site.
3GPP Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink (DL) and Discrete Fourier Transform (DFT)-spread OFDM in the uplink (UL). The basic LTE downlink physical resource may thus be seen as a time-frequency grid as generally illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized sub-frames of length Tsub-frame=1 ms, as generally illustrated in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain (see FIG. 1). Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of Virtual Resource Blocks (VRB) and Physical Resource Blocks (PRB) has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain, thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, e.g., in each sub-frame the base station transmits control information about to which user equipments data is transmitted and upon which resource blocks the data is transmitted, in the current downlink sub-frame. This control signalling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each sub-frame and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI) indicated by the Physical CFI Channel (PCHICH) transmitted in the first symbol of the control region. The control region also contains Physical Downlink Control Channels (PDCCH) and possibly also Physical Hybrid-Automatic Repeat Request (HARQ) Indication Channels (PHICH) carrying Acknowledgement/Non-Acknowledgement (ACK/NACK) for the UL transmission.
The downlink sub-frame also contains Common Reference Symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with 1 out of 3 OFDM symbols as control is generally illustrated in FIG. 3.
In order to preserve the orthogonality in UL the UL transmissions from multiple UEs the UEs need to be time aligned at the eNodeB. Since UEs may be located in a served cell at different distances from the eNodeB, see FIG. 4, the UEs will need to initiate their UL transmissions at different times. A UE far from the eNodeB needs to start transmission earlier than a UE close to the eNodeB. This can for example be handled by time advance of the UL transmissions, a UE starts its UL transmission before a nominal time given by the timing of the DL signal received by the UE. This concept is illustrated in FIG. 5.
The UL Timing Advance (TA) is maintained by the eNodeB through timing advance commands to the UE based on measurements on UL transmissions from that UE.
Through timing advance commands, the UE is ordered to start its UL transmissions earlier or later. This applies to all UL transmissions except for random access preamble transmissions on Physical Random Access Control Channel (PRACH), i.e. including transmissions on Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH), and Sounding Reference Signal (SRS).
There is a strict relation between DL transmissions and the corresponding UL transmission. Examples of this are:                the timing between a Downlink Shared Channel (DL-SCH) transmission on Physical Downlink Shared Channel (PDSCH) to the HARQ ACK/NACK feedback transmitted in UL (either on PUCCH or PUSCH);        the timing between an UL grant transmission on Physical Downlink Control Channel (PDCCH) or enhanced PDCCH (ePDCCH), also sometimes denoted evolved PDCCH, to the UL-SCH transmission on PUSCH.        
By increasing the timing advance value for a UE, the UE processing time between the DL transmission and the corresponding UL transmission decreases. For this reason, an upper limit on the maximum timing advance has been defined by 3GPP in order to set a lower limit on the processing time available for a UE. For LTE, this value has been set to roughly 667 us which corresponds to a cell range of 100 km. Note that the TA value compensates for the round trip delay.
In LTE Rel-10 there is only a single timing advance value per UE and all UL cells are assumed to have the same transmission timing. The reference point for the timing advance is the received timing of the primary DL cell.
In LTE Rel-11 different serving cells used by the same UE may have different Timing Advance (TA) values. Most likely the serving cells sharing the same TA value (for example depending on the deployment) will be configured by the network to belong to a so called TA group. If at least one serving cell of the TA group is time aligned, all serving cells belonging to the same group may use this TA value. To obtain time alignment for a Secondary Cell (SCell) belonging to a different TA group than the Primary Cell (PCell), the current 3GPP assumption is that network initiated random access (RA) may be used to obtain initial TA for this SCell (and for the TA group the SCell belongs to).
In LTE, as in any communication system, a UE (mobile terminal) may need to contact the network, via the eNodeB, without having a dedicated resource in the Uplink (from UE to base station i.e. eNodeB in this example). To handle this contact, a Random Access (RA) procedure is available where a UE that does not have a dedicated UL resource may transmit a signal to the eNodeB. The first message of this procedure is typically transmitted on a special resource (channel) reserved for RA i.e. a PRACH. This channel can for instance be limited in time and/or frequency (as in LTE). See FIG. 6 explaining the PRACH implementation in available radio block resources.
The resources available for PRACH transmission is provided to the terminals as part of the broadcasted system information in system information block 2 (SIB-2) (or as part of dedicated Radio Resource Control (RRC) signalling in case of e.g. handover).
The resources consist of a preamble sequence and a time/frequency resource. In each cell, there are 64 preamble sequences available. Two subsets of the 64 sequences are defined, where the set of sequences in each subset is signalled as part of the system information. When performing a (contention-based) random-access attempt, the terminal selects at random one sequence in one of the subsets. As long as no other terminal is performing a random-access attempt using the same sequence at the same time instant, no collisions will occur and the attempt will, with a high likelihood, be detected by the eNodeB.
In LTE, the random access procedure can be used for a number of different reasons. Among these reasons are:                Initial access (for UEs in the RRC_IDLE state).        Incoming handover.        Resynchronization of the UL.        Scheduling request (for a UE that is not allocated any other resource for contacting the base station).        Positioning.        
The contention-based random access procedure used in LTE Rel-10 is illustrated in FIG. 7. The UE starts the random access procedure by randomly selecting one of the preambles available for contention-based random access. The UE then transmits the selected random access preamble on the PRACH to eNodeB in the LTE RAN.
The RAN acknowledges any preamble it detects by transmitting a random access response (MSG2) including an initial grant to be used on the uplink shared channel, a Temporary Cell Radio Network Temporary Identifier (TC-RNTI), and a time alignment update based on the timing offset of the preamble measured by the eNodeB on the PRACH. The MSG2, i.e. the RAR, is transmitted in the DL to the UE using the PDSCH, and its corresponding PDCCH message that schedules the PDSCH contains a Cyclic Redundancy Check (CRC) which is scrambled with the RA-RNTI.
When receiving the response the UE uses the grant to transmit a message (MSG3) that in part is used to trigger the establishment of radio resource control and in part to uniquely identify the UE on the common channels of the cell. The timing alignment command provided in the RAR is applied in the UL transmission in MSG3.
In addition, the eNB can also change the resources blocks that are assigned for a MSG3 transmission by sending an UL grant that has its CRC scrambled with the TC-RNTI which was included in MSG2. In this case the PDCCH is used, to transmit the Downlink Control Information (DCI) containing the uplink grant.
The MSG4 which is then contention resolving has its PDCCH CRC scrambled with the C-RNTI if the UE previously has a C-RNTI assigned. If the UE does not have a C-RNTI previously assigned, then the UE will have its PDCCH CRC scrambled with the TC-RNTI obtained from MSG2. In the first case it is considered that the UE has included its C-RNTI into the MSG3 message whereas in the latter case the UE has included a core network identifier into the MSG3 message.
The procedure of FIG. 7 ends with the RAN solving any preamble contention that may have occurred for the case that multiple UEs transmitted the same preamble at the same time. This can occur since in contention based RA each UE randomly selects when to transmit and which preamble to use. If multiple UEs select the same preamble for the transmission on RACH, there will be contention between these UEs that needs to be resolved through the contention resolution message (MSG4). The case when contention occurs is illustrated in FIG. 8, where two UEs transmit the same preamble, p5, at the same time. A third UE also transmits at the same RACH, but since it transmits with a different preamble, p1, there is no contention between this UE and the other two UEs.
The UE may also perform non-contention based RA. A non-contention based RA or contention free RA can e.g. be initiated by the eNodeB to get the UE to achieve synchronisation in UL. The eNodeB initiates a non-contention based RA either by sending a PDCCH order or indicating it in an RRC message. The later of the two is used in case of Handover (HO).
The eNodeB can also order the UE, through a PDCCH message, to perform a contention based random access. The procedure for this is illustrated in FIG. 7. The RA order may illustrate a first step of a procedure for the UE to perform contention free random access. Similar to the contention based random access the MSG2 is transmitted in the DL to the UE and its corresponding PDCCH message CRC is scrambled with the RA-RNTI. The UE considers the contention resolution successfully completed after it has received MSG2 successfully.
For the contention free random access as for the contention based random access does the MSG2 contain a timing alignment value. This enables the eNodeB to set the initial/updated timing according to the UEs transmitted preamble.
Following is an explanation of the PDCCH monitoring procedure. A UE monitors a common search space and a UE specific search space in the PDCCH. In each search space, a limited number of candidates or equivalently PDCCH transmission hypothesis is checked, in every DL sub-frame. These are known as blind decodes, and the UE checks whether any of the transmitted DCI messages is intended for it.
The UE monitors the following RNTI that are associated with the random access and paging procedures for each associated search spaces on PDCCH:                the RA-RNTI for MSG2 is monitored in the common search space                    the TC-RNTI for MSG3 is monitored in the common search space, for reallocating the MSG3 in frequency.                        the TC-RNTI for MSG4 is monitored in the common search and UE specific TC-RNTI search space        the C-RNTI for MSG4 is monitored in the common search and UE specific C-RNTI search space.        The P-RNTI is monitored is monitored in the common search space.        
In LTE 3GPP Rel.11 discussions an enhanced PDCCH (ePDCCH) is introduced which is based on UE specific reference signals and is localized in frequency as opposed to the PDCCH which spans the whole bandwidth. Hence, a subset of the available RB pairs in a sub-frame is configured to be used for ePDCCH transmissions.
The use of UE specific precoding means that precoding gains can be achieved also for the control channels. Another benefit is that different RB pairs for ePDCCH can be allocated to different cells or different points within a cell. Thereby, Inter-Cell Interference Coordination (ICIC) between control channels may be achieved. This frequency coordination is not possible with the PDCCH since the PDCCH spans the whole bandwidth.
FIG. 9 shows an ePDCCH which, similarly to the Control Channel Element (CCE) in the PDCCH, which is divided into multiple enhanced REGs (eREGs) and enhanced CCE (eCCE) and which are mapped to one of the enhanced control regions i.e. mapped to one of the enhanced control regions/PRB pair reserved for ePDCCH transmission, to achieve localized transmission. For PDCCH, one CCE corresponds to 36 Resource Elements (RE) divided into 9 RE groups (REGs). However, the relation between eCCE and eREGs and REs is not really decided for yet in 3GPP. One proposal is that the relation between eCCE and eREGs/REs is to have them similar as for PDCCH i.e. one eCCE corresponds to 36 REs divided into 9 eREGs each comprising 4 REs that is. Another proposal is to have one eCCE corresponding to up to 36 REs and wherein each eREG corresponds to 18 REs. 3GPP may also decide that the eCCE should correspond to even more than 36 REs such as 72 or 74.
Even if the enhanced control channel enables UE specific precoding and such localized transmission as illustrated in FIG. 9, it may in some cases be useful to be able to transmit an Enhanced Control Channel (ECC) in a broadcasted, wide area coverage fashion. This is useful if the eNodeB, also sometimes denoted eNB, does not have reliable information to perform precoding towards a certain UE. Then a wide area coverage transmission is more robust, although the precoding gain is lost. Another case is when the particular control message is intended for more than one UE. In this case, UE-specific precoding can not be used. An example is the transmission of common control information as in the PDCCH (i.e. in the common search space).
In yet another case, sub-band precoding may be utilized. Since the UE estimates the channel in each RB pair individually, the eNodeB can choose different precoding vectors in the different RB pairs, if the eNodeB has such information that the preferred precoding vectors are different in different parts of the frequency band. In any of these cases, a distributed transmission may be used. FIG. 10 illustrates how eREGs belonging to the same ePDCCH are distributed over the enhanced control regions.
A UE may be configured to monitor its control channel in the ePDCCH instead of the PDCCH. Hence, both its UE-specific Search Space (USS) and its Common Search Space (CSS) are monitored in the ePDCCH resources. Alternatively, a UE may monitor the USS in the ePDCCH and the CSS in the PDCCH.
For some UE categories in the future, such as low cost Machine Type Communication (MTC) UEs, they don't monitor the PDCCH at all. One reason could be that they have a reduced and UE specific reception bandwidth and cannot receive the full system bandwidth, which is required to monitor the PDCCH. Therefore, these UEs must always monitor CSS and USS in the ePDCCH.
For at least these UEs, initial access to a cell must also be performed directly to ePDCCH. Also, UEs that are capable of monitoring either or both of PDCCH and ePDCCH may choose to perform initial access using the ePDCCH if it is available in the cell. An example given here may be wherein a UE1 and a UE2 are synchronized to an eNodeB and wherein UE1 is configured to monitor the CSS in PDCCH while UE2 is configured to monitor the CSS in ePDCCH. Under some circumstances that are discussed here below, the eNodeB is unaware of which channel a given UE is monitoring and this is a problem.
Such circumstances are when a UE:                Access the network initially, or        is performing a contention based random access to provide sync at hand over to a new cell, or        performing a contention based random access to perform scheduling request if no scheduling request resource has been assigned then the network can not distinguish whether the UE monitors the control channel, and particularly the CSS, in ePDCCH or the PDCCH since        legacy UEs only monitor the RA-RNTI for MSG2 in CSS in the PDCCH.        Rel-11 and beyond UEs may monitor the RA-RNTI for MSG2 in CSS in the ePDCCH.        
Hence when transmitting RACH MSG2 or a re-scheduling of RACH MSG3, the network does not know if PDCCH or ePDCCH should be used to communicate with the UE. It is therefore a problem how to communicate with a UE in this case.
Thus, there are problems of how to communicate with UEs in the wireless communications network and how to maintain/provide backward compatibility to be able to serve UEs of older versions.